There are a lot of sliding portions such as a chain, a gear and a bearing inside a tenter used for manufacturing an optical film and a food-packaging film. Since a lubricating oil used in such sliding portions is exposed to high temperatures, an amount of evaporation of the lubricating oil significantly affects a life time of a device. In other words, under such high temperatures, since the lubricating oil loses an inherent viscosity to form a thin film, it is necessary to restrain the amount of evaporation of the lubricating oil in order to keep lubricity thereof. In order to restrain the amount of evaporation, a high-molecular and highly viscous lubricating oil has been typically used for high-temperature applications.
However, such a lubricating oil causes a large power loss although having a small amount of evaporation, which makes overall performance of the lubricating oil unfavorable. Moreover, when such a lubricating oil is exposed to high temperatures while forming a thin film, the lubricating oil becomes solid though a large amount of residue remains. Thus, the lubricating oil not only loses characteristics as a liquid but also blocks a flow of the lubricating oil in a form of a solid sludge, which causes a poor lubrication of the sliding portions. Although such a disadvantage can be simply solved by increasing the amount of the lubricating oil in use, such a solution is not favorable in terms of costs and an environmental aspect. Consequently, as the lubricating oil used under high temperatures, a lubricating oil whose evaporation amount under high temperatures is restrained and whose fluidity is kept for a long time has been demanded. Moreover, in a tenter for manufacturing an optical film, a food-packaging film, a film for a solar battery panel and the like, since scattering of the lubricating oil on a product is extremely disfavored, reduction of the amount of the lubricating oil in use has also been demanded. Further, reduction of electrical power required for operating the device has also been demanded in terms of energy saving.
Accordingly, as such a lubricating oil for high-temperature applications, there has been proposed a lubricating oil composition that contains a polyol ester synthetic oil and a diphenylamine derivative having a C12-C72 fatty acid and/or an aryl alkyl group having a number average molecular weight of 400 to 800 (see Patent Literature 1).